Though sales rose 18% to $54.5 billion in the last quarter of 2012, Wall Street analysts seized on the company's weaker-than-expected growth in sales of iPhones, which totaled 47.8 million units, less than the high end expectation of about 50 million. But trouble may also be brewing for Apple's vaunted music service.

Quarterly revenues for iTunes, software and services totaled $3.7 billion, up from $3 billion a year ago. The gain was less than many analysts expected, including Walter Piecyk of BTIG, though the details of Apple's music sales may be partly obscured by reclassification of certain line items on its earnings report. Regardless, Apple is quietly starting to look a little less invincible on the iTunes front, just as it is with the iPhone.

"Five years ago, the iTunes store was perceived as something that would be a differentiator and something that would make an iPhone customer sticky," says Piecyk. "To me, that seems like saying, 'Well, you're going to keep your record player because you've got this huge collection of records. Or tapes.'"

There's no denying the trend towards streaming. Though it's difficult to quantify, some industry sources believe that services like Pandora, Spotify and Rdio already account for the majority of total music consumption. As that continues to become the norm for more and more music fans, the idea of having a collection may soon be as outdated as the cassette.

This poses problems for Apple, whose central strategy for music in the streaming world has involved the concept of having a collection. The company's iTunes match allows users to access their mp3 libraries from any device, played at a reasonably good sound quality, no matter where the songs were downloaded. A model that pre-supposes interest in owning music could be a dangerous one going forward.

"People have built up libraries," says Piecyk. "But the functionality of something like Spotify and the fact that it works across multiple devices reduces the interest in buying songs through iTunes and reduces that as a point of differentiation for Apple."

One potential solution: Apple's long-rumored "Pandora-killer" streaming radio service. Still, such a product would encounter plenty of competition in an already-crowded marketplace.

In the end, iTunes revenues make up only 9% of Apple's total sales, compared to 56% for the iPhone. But the company would be wise to continue watching both closely as the music industry it quickly conquered continues to evolve.